Comparison of physicochemical, sorption and electrochemical properties of nitrogen-doped activated carbons obtained with the use of microwave and conventional heating

A series of new nitrogen-doped activated carbons has been obtained via the reaction with urea and chemical activation of Polish brown coal. In order to obtain nitrogen groups bonded in different ways to the carbonaceous matrix, the modification with urea was performed at two different stages of processing, i.e. precursor or char. Additionally, the effects of conventional or microwave heating on the physicochemical parameters, sorption abilities as well as capacitance behaviour of the carbons prepared were tested. All the materials under investigation were characterized by elementary analysis, surface area measurements as well as estimation of the number of surface functional groups. The sorption properties of the materials were tested towards methylene blue at temperature of 25 °C. Moreover, symmetric supercapacitors containing organic electrolyte and prepared carbons were tested in Swagelok® type cells by using CV, GCD and EIS methods. Depending on the variant of preparation, the final products were micro/mesoporous nitrogen-doped activated carbons of well-developed surface area ranging from 617 to 1117 m2/g, showing acidic or intermediate acid-base character of the surface and different contents of nitrogen functional groups varying from 1.0 to 5.6 wt%. The results obtained in this study showed that introduction of nitrogen and chemical activation of brown coal led to activated carbons with very good sorption capacity toward organic dyes as well as good electrochemical parameters. The specific capacitance was achieved up to 86 F/g for N-doped carbon obtained by microwave carbonization followed by chemical activation with potassium carbonate.